As various electric devices, notably cellular phones and the like, have been reduced in size and thickness, loudspeakers built into such electric devices are also desired to be smaller and thinner. In such a situation, a loudspeaker having a dome-shaped diaphragm as shown in FIG. 7 is known as one that realizes not only reduction in size and thickness but also excellent sound quality.
The loudspeaker in the figure includes a frame yoke 100 made of a magnetic material and centrally provided with a bottomed cylinder 110, a columnar magnet 200 that is fixedly attached to the bottom of the bottomed cylinder 110, and a disk shaped pole piece 300 that is fixedly attached to the upper surface of the magnet 200. The bottomed cylinder 110, the magnet 200, and the pole piece 300 constitute a magnetic circuit of the loudspeaker. An annular magnetic gap G is formed in a space between the inner circumferential surface of the bottomed cylinder 110 and the outer circumferential surface of the pole piece 300.
The conventional loudspeaker has a circular dome-shaped diaphragm 500 made of polyethylene terephthalate (PET) resin and fixedly attached to an outer edge portion of the frame yoke 100. In order to increase rigidity, this diaphragm has a double-layered structure excluding its outer circumferential edge. A cylindrical voice coil 400 is attached to the lower surface of the dome-shaped diaphragm 200. In this state, the voice coil 400 is positioned within the magnetic gap G. The dome-shaped diaphragm 500 and the voice coil 400 constitute a vibration system of the loudspeaker.
In the loudspeaker having the above-described structure, when amplified audio signals are inputted to the voice coil 400 via a terminal not shown, electromagnetic actions are produced between the voice coil 400 and the magnet 200 to cause vibrations of the dome-shaped diaphragm 500 and the voice coil 400, and thus sounds according to the audio signals are reproduced.
General requirements for a diaphragm of a loudspeaker reproducing high-quality sound are lightweight, high rigidity, and a moderate level of internal loss. To meet these requirements, Japanese Patent No. 3643855 discloses a diaphragm of a double-layered structure, Japanese Unexamined Patent Publication Nos. 52-93317 and 2005-110092 each disclose a diaphragm in which a thin film is formed over a film base.